The term electrochromic is broad and encompasses within it various media, such as, for example, solution-phase, surface-confined, electro-deposition, or combinations thereof, which undergo a change in transmittance to light, and a concomitant change in color, when an electrical potential difference is imposed across the electrochromic media in a device.
There has been a great deal of research on surface-confined electrochromic media where the layers changing their transmittance to light are thin films deposited on the transparent electrodes within an electrochromic device. In these devices, an anodic electrochromic layer and a cathodic electrochromic layer are separate and distinct, and are electrically connected by a conductive electrolyte. Either thin film may be stoichiometric and nonstoichiometric forms of transition metal oxides, such as for example tungsten oxide, molybdenum oxide, nickel oxide, rhodium oxide, iridium oxide, niobium oxide, vanadium oxide, titanium dioxide, and combinations thereof. These electrochromic solid films are typically paired with an auxiliary redox system, which may be another metal oxide, or may be a solution phase, a surface-confined polymer film, or an electro-deposited polymer film species (all of which are described in detail hereinbelow).
These surface-confined thin films may also be a polymeric layer such as polyanaline, polypyrroles, polythiophenes, and the like. U.S. Pat. No. 5,282,955, entitled "Electrically Conductive Polymer Composition, Method of Making Same and Device Incorporating Same" to N. Leventis et al. teaches an electrically conductive polymer with a porous structure having an electrochromic compound coated on the surfaces of the pores of the structure.
The electrochromic media may comprise an electro-deposition-type materials such as, for example, metal, metal oxides and heptyl viologen bromide in water.
Finally, the electrochromic media may include a solution-phase system. Solution-phase electrochromic devices and various circuitry and applications thereof are described in U.S. Pat. No. 4,902,108, entitled "Single-Compartment, Self-Erasing, Solution-Phase Electro-optic Devices Solutions for Use Therein, and Uses Thereof", issued Feb. 20, 1990 to H. J. Byker; Canadian Patent No. 1,300,945, entitled "Automatic Rearview Mirror System for Automotive Vehicles", issued May 5, 1992 to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled "Variable Reflectance Motor Vehicle Mirror", issued Jul. 7, 1992 to H. J. Byker; U.S. Pat. No. 5,202,787, entitled "Electro-Optic Device", issued Apr. 13, 1993 to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled "Control System For Automatic Rearview Mirrors", issued Apr. 20, 1993 to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled "Tinted Solution-Phase Electrochromic Mirrors", issued Jan. 11, 1994 to D. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled "UV-Stabilized Compositions and Methods", issued Jan. 18, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled "Variable Reflectance Mirror", issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled "Variable Reflectance Mirror", issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No. 5,294,376, entitled "Bipyridinium Salt Solutions", issued Mar. 15, 1994 to H. J. Byker; U.S. Pat. No. 5,336,448, entitled "Electrochromic Devices with Bipyridinium Salt Solutions", issued Aug. 9, 1994 to H. J. Byker; U.S. Pat. No. 5,434,407, entitled "Automatic Rearview Mirror Incorporating Light Pipe", issued Jan. 18, 1995 to F. T. Bauer et al.; U.S. Pat. No. 5,448,397, entitled "Outside Automatic Rearview Mirror for Automotive Vehicles", issued Sep. 5, 1995 to W. L. Tonar; and U.S. Pat. No. 5,451,822, entitled "Electronic Control System", issued Sep. 19, 1995 to J. H. Bechtel et al. Each of these patents is commonly assigned with the present invention and the disclosures of each, including the references contained therein, are hereby incorporated herein in their entirety by reference. Additionally, the following references by others are also incorporated herein in their entirety by reference: U.S. Pat. Nos. 3,806,229 and 3,451,741; European Patent Application Publication Nos. 0 012 419, 0 430 684, 0 430 686, 0 435 689 and 0 552 012; and Non-emissive Electrooptic Displays, Kmetz and von Willisen, eds., Plenum Press, New York, N.Y., USA (1976), and especially the chapter therein by Chang, "Electrochromic and Electrochemichromic Materials and Phenomena," at pp. 155-196.
In typical solution-phase electrochromic devices, and particularly devices which are single-compartment and self-erasing, a solution is held as a thin layer in a compartment which is formed by two walls, at least one of which is transparent to light (electromagnetic radiation of wavelength in the visible range), and spacers or sealant which separate the two walls and form the periphery of the compartment. The inner sides, those which face each other, of the two walls are each coated with an electrode layer which is in contact with the solution. An electrode layer functions as an electrode in contact with the solution and is a layer of a material which is electronically conducting. The electrode layer on at least one of the walls is transparent to light, because, as indicated above, at least one of the walls is transparent to light. Transparent electrode layers may be made of tin oxide, tin-doped indium oxide, indium tin oxide, fluorine-doped tin oxide, fluorine-doped zinc oxide, gold, cadmium stannate, ruthenium oxide, or the like, as known in the art. One of the walls and, consequently, one of the electrode layers may be non-transparent. For example, a non-transparent electrode layer might be a reflecting layer, a layer which reflects light, and may be made of a metal, semiconductor material, or the like which may or may not be specularly reflecting.
The layer of solution or other type of medium between the walls of an electrochromic device is sometimes referred to as an "electrochromic layer."
When a sufficient potential difference is applied between the electrode layers across the solution of such a device, the transmittance of the solution changes at least one wavelength in the visible range and, as a consequence, the solution changes color, becoming darker or clearer. Typically, the solution in such a device will be clear or slightly colored (tinted) in its zero-potential, equilibrium state and will be darkened through electrochemical reaction(s) when a potential difference is applied. If the device is a solution-phase electrochromic device, the electrochromic compounds (those which have a change in transmittance in the visible wavelength range upon electrochemical oxidation (anodic electrochromic compound) or reduction (cathodic electrochromic compound) are in solution and remain in solution without precipitation upon oxidation or reduction in operation of the device.
In a single-compartment device, at least one anodic electrochromic compound and at least one cathodic electrochromic compound are together in the same compartment and are able to diffuse throughout the entire compartment (e.g., layer between the electrode layers).
In the case of a single-compartment device, self-erasing occurs, when there is no potential difference between the electrode layers, as oxidized anodic compound and reduced cathodic compound react with one another by electron transfer and both return to their zero-potential equilibrium states. However, an electrochromic device need not have both the anodic and cathodic electrochromic compounds in the solution. One compound in solution may be paired with a surface-confined material or an electro-deposited material.
Solutions of variable transmittance in solution-phase electrochromic devices may comprise components in addition to solvent and electrochromic compounds. Such components may include inert, current-carrying electrolyte(s), thickening agents (such as, for example, non-cross-linked polymers like polymethylmethacrylate), tinting agents and UV-stabilizing agents. UV-stabilizing agents inhibit degradation of components of an electrochromic layer upon exposure of the layer to ultraviolet (UV) radiation.
The '108 Patent, among others, describes certain advantages realized by thickening or gelling solutions used in single-compartment, self-erasing, solution-phase electrochromic devices. One of the problems associated with such devices is that of segregation. When operated continuously for long periods of time, the oxidized form of the anodic and reduced form of the cathodic electrochromic materials in such devices tend to segregate. Gelling or thickening the solutions of the electrochromic device reduces the component of segregation that is due to natural convection of the electrochromic medium, thereby reducing the extent of segregation and its undesirable effects, such as uneven coloring or clearing.
Thickening or gelling the solution in electrochromic devices also creates the advantages of slower spreading of solution, restricted shattering and easier clean-up in the case of breakage of the device.
Electrochromic solutions gelled or thickened through the use of materials, such as colloidal silica or acrylic fibers, which do not involve covalent cross-linking of polymer chains, have been described. See Manos, U.S. Pat. No. 3,451,741; Shelepin et al., USSR Patent Publication No. 566,863; and the '108 Patent.
Thickened or gelled electrochromic solutions in the art suffer from a number of shortcomings that have restricted or prevented the practical application of electrochromic devices to provide variable transmittance or variable reflectance in a number of contexts. Perhaps the most important of these contexts is apparatus, such as windows or large outside rearview motor vehicle mirrors, where devices with solution layers of large area, more than about 0.1 m on a side, are oriented nearly vertically (i.e., nearly parallel to the lines of force of the gravitational field of the Earth) or are otherwise subjected to conditions which entail significant hydrostatic pressure and concomitant large forces pushing outwardly from the solution against the walls of the device. Thus, in these large area apparatus, hydrostatic pressure makes solution-phase electrochromic devices susceptible to breakage, for example due to rupture of seals holding walls of the electrochromic device together. Even when there is not breakage, the hydrostatic pressure causes bowing out of the walls of the electrochromic device, which results in non-uniform thickness in the solution layer and undesirably non-uniform coloring and clearing during operation of the device.
Solutions thickened by prior art methods, although thickened to the point of reducing flow, are not "free-standing" or permanent gels (see Sperling, Introduction to Physical Polymer Science, John Wiley & Sons, Inc., New York, N.Y., 2 nd ed. (1992)). In free-standing (permanent) gels, solution is interspersed and entrapped in a polymer matrix and continues to function as a solution. Because solutions thickened by prior art methods (e.g., Shelepin et al., supra; '108 Patent) are not free-standing gels, the fluid in them is not entrapped in a polymer matrix and, consequently, still exerts undesirable hydrostatic pressure and concomitant device-breaking or device-distorting forces in large area devices.
However, the use of free-standing (permanent) gels to create a workable electrochromic device is not trivial. To create a free-standing gel some amount of polymerization and/or crosslinking is necessary and, generally when polymerization and/or crosslinking takes place, the volume of the crosslinked polymer is smaller than the pre-crosslinked monomers. This polymerization and crosslinking can take place substantially simultaneously in an electrochromic device, i.e., in situ polymerization. However, there are limitations with in situ polymerization. To obtain a free-standing gel through in situ polymerization, some of the monomers must have a functionality greater than two. If polymerization and crosslinking takes place in the device, there is a significant amount of shrinkage in the polymer solution. This shrinkage causes the solid polymer to crack, craze and form voids, all of which adversely affects the usefulness of the final device. Furthermore, the detrimental effects will sometimes not be noticed for some time since polymerization and crosslinking can occur over a period of weeks. If, on the other hand, only polymerization, and no crosslinking takes place, a "free standing" polymer will not form and hydrostatic pressure will build up and adversely affect the operability of the final device. Therefore, it is important to have sufficient polymerization and crosslinking to create a free-standing gel while avoiding a significant amount of shrinkage.
This polymerization and/or crosslinking can be initiated by chemical-, thermal- or UV-type initiators. A common method of UV curing can be accomplished by adding a constituent that, when exposed to UV light, will form a radical to initiate polymerization and/or crosslinking. Difficulties arise with UV curing because not all the radicals are consumed to initiate a reaction and/or self-react and are therefore present in the device after "final cure" of the polymer gel. These radicals may also be later induced into their reactive state by subsequent UV radiation experienced by a typical electrochromic device during its lifetime. As more reactive species are produced, further polymerization and/or crosslinking will occur thereby changing the gel properties and further degrading the gel. This is especially true in UV stabilized devices which utilize UV absorbing materials. In addition, the electrochromic materials may interfere with light absorption or initiation and may inhibit or retard the polymerization process. Thus UV curing is not presently preferred.
An electrochromic solution gelled or thickened with the use of a covalently cross-linked polymer matrix of 1-vinyl-2-pyrrolidinone-co-N,N'-methylbisacrylamide and used in providing color to an electrochromic device has been described. Tsutsumi et. al., J. Polymer Sci. A, 30, 1725-1729 (1992).
It would clearly be desirable, then, to provide, as media of reversibly variable transmittance for electrochromic devices, electrochromic layers that behave as free-standing gels and that, as such, do not flow at perceptible rates and do not "weep" or exude liquid (leak fluid by syneresis, see Sperling, supra) but retain functional characteristics of a solution allowing for diffusion of electrochromic species. It would be especially desirable if such a layer would adhere to the electrode layers on the walls of such a device to further counteract separation of the electrode layers and walls. Further, it would be desirable if such a layer would be tough and rubbery and behave similarly to the lamination layer in laminated safety glass. Such electrochromic layers would substantially eliminate problems presented by hydrostatic pressure and the concomitant forces when solutions, and even conventionally thickened solutions, are used to provide variable transmittance in electrochromic devices. Finally, it would be desirable if such a layer would not shrink to such an extent that the usefulness of the final device is compromised.
However, electrochromic layers which would have such favorable structural, flow and electrode-layer-adherence properties would be chemically complex. Consequently it is not straightforward to provide such an electrochromic layer that retains other characteristics that are important for practical applications of media of reversibly variable transmittance in electrochromic devices, especially such devices which are desirably solution-phase, single-compartment and self-erasing.
These other characteristics, which are necessary or important for practical applications of electrochromic layers, include, without limitation, the following. Such a layer, particularly in applications such as automobile mirrors or automobile or airplane windows, should not shrink significantly when polymerized and/or crosslinked, separate into solid and fluid phases, weep, or sag when subjected to vibration. Weeping and sagging should also be avoided in other applications, such as interior or exterior building windows. The electrochromic layer should not be hazy or cloudy. The compositions, which are part of the electrochromic layer and afford the layer its favorable structural, flow and adherence characteristics, must not be involved in undesirable interactions with other components of the layer. For example, the cycle life of the layer should not be seriously degraded by constituents of the polymer matrix. This is very important in the field of electrochromics where even small impurities can significantly influence the successful operation of a device. Also, if an initiator or a catalyst is employed in making a polymer that is part of the layer, the initiator or catalyst should not undergo reactions with electrochromic compounds in the layer, in their zero-potential equilibrium or electrochemically activated (reduced or oxidized) states. If a polymer is part of the electrochromic layer and involved in an interaction with the electrode layers that causes the electrochromic layer to adhere to the electrode layers, the polymer should not significantly interfere with the function of the electrode layer in electron transfer to or from electrochromic species in the layer. The compositions which provide the desirable structural, flow and adherence characteristics to the electrochromic layer should not cause the layer to be so unstable to ultraviolet (UV) radiation that the layer cannot be made sufficiently stable by other measures, such as addition of UV-stabilizing agents to the solution in the layer. The compositions which provide the desirable structural, flow and adherence characteristics to the electrochromic layer should not interfere with the coloring and clearing times of a device to an extent that would render use of the device impractical. The compositions which provide the desirable structural, flow and adherence characteristics to the electrochromic layer should not significantly complicate construction or assembly of an electrochromic device in which the layer provides reversibly variable transmittance. Thus, for example, it would be desirable to be able to fill a device with all components of an electrochromic layer, including those that afford the favorable structural, flow and adherence characteristics, before the layer loses its ability to flow. Finally, the reactions to provide an electrochromic layer its favorable structural, flow and adherence properties should be completed soon after the reactions are started. For example it is undesirable to have significant polymer formation continuing in an electrochromic layer inside a device for more than a few days, because the layer may shrink with time or the performance characteristics (e.g., color in the clear state, coloring and clearing times, uniformity of coloring) of the layer and the device that includes the layer would then undesirably change over time.
The present invention fills a need in the art for an electrochromic layer which has the advantageous structural, flow and electrode-layer-adherence properties described above for use of the layer to provide variable transmittance or reflectance in a large-area electrochromic device and, at the same time, has other characteristics which make the layer acceptable for practical applications in single-compartment, self-erasing electrochromic devices that also function as solution-phase devices.